It is a convenience for a wrench to have a mechanism enabling the wrench to releasably lock onto a nut or bolt. For example, where a nut is to be threaded onto a bolt in an awkward or inaccessible position, the job is greatly simplified if the nut can be held in a wrench without being free to fall out, at least while placing the nut on the bolt and during the initial stage of threading. The present invention provides a wrench having this facility and which also has a simple-to-operate locking and release mechanism.